1. Field of the Invention
The present invention relates to a connecting rod made of a titanium alloy, and more particularly, the present invention relates to a split-type connecting rod made of a titanium alloy having a split big end. Moreover, the present invention also relates to an internal combustion engine and an automotive vehicle incorporating such a connecting rod.
2. Description of the Related Art
In an internal combustion engine of an automotive vehicle, a part which is called a connecting rod (or con rod) is used for linking a crankshaft to a piston. FIG. 46 shows a conventional connecting rod 401. The connecting rod 401 includes a bar-like rod main body 410, a small end 420 which is provided at one end of the rod main body 410, and a big end 430 which is provided at the other end of the rod main body 410.
The small end 420, which has a throughhole (piston pin hole) 425 for allowing a piston pin to extend therethrough, is connected to a piston. On the other hand, the big end 430, which has a throughhole (crankpin hole) 435 for allowing a crankpin to extend therethrough, is connected to a crankshaft.
The big end 430 is split into a rod portion 433 which continues from one end of the rod main body 410, and a cap portion 434 which is coupled to the rod portion 433 with bolts 440. FIG. 47 shows a connecting rod 401 without the bolts 440 being engaged. In the big end 430, bolt holes 432 are formed so as to penetrate through both the rod portion 433 and the cap portion 434, thus allowing the bolts 440 to be screwed into the bolt holes 432.
The connecting rod 401 shown in FIG. 46 and FIG. 47 is called a split-type connecting rod because its big end 430 is split into the rod portion 433 and the cap portion 434 as described above.
Conventionally, steel has widely been used as the material used for forming connecting rods. In recent years, use of a titanium alloy has been proposed in order to reduce the weight of the connecting rod (see, for example, Toshihiko MATSUBARA, “Development of Free Machining Titanium Alloy for Connecting Rods”, Titanium Zirconium, October 1991, Vol. 39, 4th issue, pp. 175-184). However, since a titanium alloy has different material characteristics from those of steel, the following problems may occur when producing a split-type connecting rod by using a titanium alloy.
The modulus of elasticity (i.e., the Young's modulus) of a titanium alloy is around 100 GPa to 110 GPa, which is about half of that of steel. Therefore, under the same amount of stress, a titanium alloy will be strained about twice as much as steel. Therefore, in the case where a titanium alloy is used as the material of a split-type connecting rod, if it is similar in shape to a steel connecting rod, the connecting rod will have a lower rigidity and thus, its big end will experience a significant deformation during use.
A deformed big end is schematically shown in FIG. 48. As can be seen from FIG. 48, inertial force which acts on the connecting rod deforms the rod portion of the big end so as to shrink inwards, whereby the roundness of the crankpin hole is reduced. This causes problems in that the friction loss may increase, and in that a bearing metal which is placed inside the crankpin hole of the big end may stick to the crankpin.
In order to prevent such problems, a conventional connecting rod made of a titanium alloy is designed so that its big end is thicker than that of a steel connecting rod in an attempt to ensure rigidity and suppress deformation of the big end. FIG. 49 schematically shows a connecting rod made of a titanium alloy 501 which is disclosed in MATSUBARA, supra. As can be seen from FIG. 49, shoulders 531a and 531b (i.e., portions extending from the rod main body 510 toward both lateral sides) of the rod portion 533 are significantly thick.
However, increased thickness will result in an increased weight of the connecting rod, thus detracting from the effect of weight reduction that is realized by using a titanium alloy, which has a small specific gravity.